A number of oligonucleotide analogs have been made. One class of oligonucleotides that have been synthesized are the 2'-O-substituted oligonucleotides. Such oligonucleotides have certain unique and useful properties. In U.S. patent application Ser. No. 814,961, filed Dec. 24, 1991, entitled Gapped 2' Modified Phosphorothioate oligonucleotides, assigned to the same assignee as this application, the entire contents of which are herein incorporated by reference, 2' substituted nucleotides are introduced within an oligonucleotide to induce increased binding of the oligonucleotide to a complementary target strand while allowing expression of RNase H activity to destroy the targeted strand.
In a recent article, Sproat, B. S., Beijer, B. and Iribarren, A., Nucleic Acids Research, 1990, 18:41, the authors noted further use of 2'-O-methyl substituted oligonucleotides as "valuable antisense probes for studying pre-mRNA splicing and the structure of spliceosomes".
2'-O-Methyl and ethyl nucleotides have been reported by a number of authors. Robins, et al., J. Org. Chem., 1974, 39, 1891; Cotten, et al., Nucleic Acids Research, 1991, 19, 2629; Singer, et al., Biochemistry 1976, 15, 5052; Robins, Can. J. Chem. 1981, 59, 3360; Inoue, et al., Nucleic Acids Research, 1987, 15, 6131; and Wagner, et al., Nucleic Acids Research, 1991, 19, 5965;112.
Sproat, B. S. and Lamond, A. I., in "12'-O-Methyloligo-ribonucleotides: synthesis and applications, Oligonucleotides and Analogs A Practical Approach; Eckstein, F. Ed.; IRL Press, Oxford, 1991, describe syntheses of 2'-O-methylribonucleoside-3'-O-phosphoramidites. The uridine phosphoramidite synthesis described therein requires both base and sugar protection of the starting nucleoside prior to alkylation. Only after the base and sugar protecting groups are in place on the uridine is it then alkylated. Post alkylation, the base protecting group is removed followed by 5'-O-dimethoxytritylation and phosphitylation. The cytidine phosphoramidite synthesis described by Sproat and Lamond utilizes (and thus requires) the base and sugar blocked 2'-O-methyl uridine analog. This analog is then converted to a blocked cytidine analog, the blocking group is removed from the sugar, the analog is dimethoxytritylated and finally phosphitylated. The guanosine phosphoramidite synthesis taught by Sproat and Lamond starts from a 2-amino-6-chloronucleoside having 3' and 5' sugar hydroxy groups blocked. This nucleoside is converted to a 2,6-dichloro derivative. The dichloro compound is then 2'-O-alkylated. Following O-alkylation, the dichloro compound is converted to a diazido intermediate. The diazido intermediate is in turn converted to a diamino intermediate. The diamino intermediate is then deaminated to the guanosine analogue. The 2-amino group of the guanosine analogue is blocked followed by dimethoxytritylation and finally phosphitylation. This guanosine procedure is also published in Sproat, et. al., Nucleic Acids Research, 1991 19:733.
The above synthetic procedures involve multiple steps and numerous reagent treatments--9 different reagent treatments for uridine, 10 for cytidine and 12 for guanosine. For the cytidine and guanosine compounds at least one of the reagents that is required is not readily available and thus is a very expensive reagent.
Certain oligonucleotides containing 2'-O-alkyl substituted nucleotides are promising candidates for use as human pharmaceuticals. For use in large scale therapeutic testing and eventually for human pharmaceutical use, large amounts of these oligonucleotides must be synthesized. The large amounts of oligonucleotides in turn requires large amounts of the 2'-O-alkyl nucleoside phosphoramidites used in synthesizing the oligonucleotides. Consideration must therefore be given to both cost and purity of the starting phosphoramidites used in the synthesis of such oligonucleotides. As a general premise, as the number of synthetic steps increases, the cost of manufacture increases. Further as the number of steps increases, quality control problems escalate. In view of this, it is evident that there is a great need for new and improved procedures for preparing nucleoside phosphoramidites.